Conductivity

Conductivity (or specific conductance) in simple words can be described as the ability of an electrolyte solution to conduct electricity. However, all the solutions are not necessarily electrolytes therefore all the solutions will not conduct electricity. The conductivity is also mentioned as specific conductivity in scientific terms. The conductivity in the SI unit is Siemens per meter (S/m). 

The conductivity measurements are a reliable, fast, and inexpensive way of measuring ionic content in a given solution. This is the reason why it is widely adopted in industrial and environmental applications. For eg, in the water purification systems, the conductivity measurement monitoring gives you a perfect trend of the status of water quality. 

Conductivity Chemistry

The specific conductivity of a solution or electrolyte depends on the concentration of electrolytes in it. Conductivity is nothing but the measurement of the movement of electrolytic ions present in the solution. In many cases as the concentration of the electrolyte increases, after a certain peak point, it becomes neutral. Consider the sodium chloride solution which is a good conductor of electricity, but as there is saturation it becomes inactive. 

Based on strength, two types are classified as strong electrolytes and weak electrolytes. Strong electrolytes are assumed to dissolve completely in water. That means when particular quantities of these electrolytes are dissolved in water, they give peak conductance but if the quantity is increased then it starts reducing its conductance. Weak electrolytes do not dissolve completely in water which means that even the low concentrations will not dissociate fully and the conductance will be below. 

Electrical Conductivity 

From an applied electric field forces act upon the electrically charged particles which results in an electrical current. In solid materials, current results due to the flow of electrons which is called electronic conduction. Only electronic conduction exists in all conductors, semiconductors, and many insulated materials. The availability of the number of electrons to contribute to the conduction process decides the electrical conductivity. 

Metals, most of them, are the very good conductor of electricity because of higher free electrons. Electrical conductivity can be defined as the ratio between the current density (J) and the electric field intensity (e) and it is the opposite of the resistivity. Silver is said to have the highest conductivity of any metals: 63 x 106 S/m.    

The Conductivity of Water 

Pure water is not a good conductor of electricity but it increases with the increase in ionic concentrations. Typically the water of different purity shows the different conductivity as follows – 

Seawater – 5 S/m

Distilled water Conductivity – 5.5 10 - 6 S/m  

Conductivity of Drinking water – 0.005 – 0.05 S/m

Water TDS and Conductivity  

TDS (total dissolved solids) and EC are quite comparable. It is quite possible to calculate the TDS of water once you measure the conductivity of water. Thus we can know the TDS of distilled water or drinking water if we measure the conductivity of drinking water and distilled water conductivity. 

TDS is a measure of total ions in the solution and EC is a measure of the ionic activity of a solution, so it is quite relevant to each other. TDS of water can be calculated if we measure conductivity, by following the formula – 

TDS (mg/l) = 0.5 x EC (dS/m or mho/cm) or 0.5 X 1000 x EC (ms/cm)

The unit of measurement of TDS is micro siemens or ppm (parts per million)

The water conductivity meter is widely used to measure the conductivity of water. In recent times, there is no need to apply a formula to calculate TDS as the conductivity meter has the dual function of displaying both TDS and conductivity. 

As the concentration of the solution increases (EC >2000 or TDS >1000), the bonding of ions to each other increases, and this tends to decrease their activity and result in the reduction in carrying current. As TDS goes higher the ratio of TDS/EC gets saturated to TDS = 0.9 x EC. This tends to deviate from the TDS and EC relationship and the sample has to be treated differently.

The measurement of water conductivity with help of a water conductivity meter helps in a lot of fields like manufacturing, agriculture, irrigation, etc. the quality of water plays an important role in these industries to maintain the appropriate quality of the products and crops. In the case of irrigation and agriculture, the values of EC and TDS are related to each other and is converted to the accuracy of about 10% - 

TDS (mg/l) = 640 x EC (ds/m or mho/cm). In manufacturing industries such as food industries and especially in water and beverage manufacturing, the quality of water is monitored and maintained as per regulations. This also helps the manufacturers to keep their products of uniform taste all over. 

As the water parameters and taste differ from place to place the product taste also might differ vastly which will impact the brand. Thus in maintaining the uniform quality, TDS and conductivity play an important role as these are very basic characteristics of water. 

In the packaged drinking water section, the water quality is maintained by the application of the reverse osmosis process which again enhances the water quality in terms of TDS and EC. In the reverse osmosis process, the water is pressurized through a semipermeable membrane that leaves the impurities behind. With the RO process, around 95-99% of TDS is removed and gives highly purified water.